John, You have it!!
Let's face it (pun intended), a portrait that speaks beyond words is one that captured the right expression on the subjects face, usually. You must be ready when you see it and it rarely happens by chance.
With children I still have them say "cheeeese", (unless they insist on not being happy) it always works and holds their attention long enough to get the shot, but with adults I usually instruct them to look down - sometimes I offer/suggest a thought - and when they feel relaxed or ready, to look up at me or at the camera. So, yes, it's mostly about the eyes ("windows to the soul").